The present invention relates to a process for the preparation of toluylene diisocyanate (TDI). This process comprises reacting toluene with nitric acid to yield dinitrotoluene (DNT), hydrogenating the resultant dinitrotoluene (DNT) to yield toluylene diamine (TDA) and reaction water, and reacting the toluylene diamine with phosgene to give TDI. However, the process steps to form toluylene diamine (TDA) are performed in a first production plant wherein the crude solution of toluylene diamine (TDA) and water from the hydrogenation step is distilled to form an inter-mediate mixture of toluylene diamine and water containing about 1 to 40% by weight of water. This intermediate mixture is transported to a second production plant located at some distance from the first plant. In the second production plant, the distillation of the intermediate mixture of toluylene diamine (TDA) and water is completed to yield dry toluylene diamine, which is then phosgenated to yield toluylene diisocyanate (TDI).
The present invention also relates to specific mixtures of toluylene diamine and water, and to the use of these mixtures to prepare toluylene diisocyanate at a location which is different from the location at which the mixture of toluylene diamine and water has been obtained or prepared.
Normally the large-scale production of TDI is performed using the process stages of reacting toluene with nitric acid to yield DNT and water, reacting the DNT with hydrogen to yield TDA and water, followed by reacting the processed, dried TDA with phosgene to yield TDI and hydrogen chloride wherein the production units for each stage of the process are linked to each other in one production plant.
It may be advantageous, however, to perform the process steps in two production plants located at some distance from each other, wherein the process stages up to the production of TDA are performed in one production plant and the reaction of TDA with phosgene and processing to give a marketable TDI final product are performed in the second production plant. This type of procedure may be economically attractive if, for instance, suitable raw materials and infrastructures are readily available in one area, but a large purchasing market with the necessity for local production of the final product is provided in another area located a considerable distance away. Furthermore, it may also be economically advantageous to supply: a variety of small phosgene units in various locations from one central, integrated amine plant located some distance away.
This mode of production and/or operation is made considerably more difficult, however, by the fact that the intermediate product, TDA, has a high melting point. This means that TDA can be transported only either in the solid form or as a hot melt at a temperature of more than 100xc2x0 C. When transporting TDA in the solid form, the TDA first has to be subjected to an expensive processing stage such as, for example, to produce flakes, in order to be able to melt the product again after transportation for use in its final phosgenation reaction. On the other hand, overseas transport as a hot melt requires the use of heatable tank containers and appropriate heating facilities on board ship such as, for example, connections for heating steam or for electrical power, or, in the case of transportation as bulk goods, the use of tanker ships whose holds can be heated to a temperature of 105-110xc2x0 C. The first case, which requires use of heatable tank containers, is very expensive and uneconomical for the transportation of large amounts of TDA. In the second case, transportation as bulk goods in ships with heatable holds, would also be very expensive and, therefore, uneconomical because conventional tanker ships and the trans-shipment devices in docks are not intended for use at high temperatures. This method would first require the tanker ships and trans-shipment devices to be adapted for this purpose, which would be a great expense. Finally the problem of disposing of the slops from the ships"" holds, optionally, by means of expensive special waste incineration procedures on land, would have to be overcome.
U.S. Pat. No. 5,449,832 describes one process for storing and transporting toluenediamine (TDA). This process comprises dinitrating toluene to yield the 2,4- and 2,6-isomers of dinitrotoluene, hydrogenating the dinitrotoluene to yield the 2,4- and 2,6-isomers of toluenediamine, and distilling the toluenediamine to produce essentially anhydrous product of 2,4- and 2,6-toluenediamine, which is then cooled and transferred for storage and/or shipment. Prior to storing and/or shipping the toluenediamine (TDA), the melting point of the TDA is reduced by adding water in an amount of about 5 to 15%, preferably 7 to 10% by weight (based on the weight of anhydrous TDA), and controlling the temperature of the resultant TDA-water mixture such that the final temperature of the TDA-water mixture is at or below the boiling point. It is this mixture which is suitable for long term storage and/or transport. The water added to the anhydrous TDA is hot demineralized water, a deionized water, or distilled water under pressure. The final temperature level from the addition of water to the anhydrous TDA provides sufficient internal heat to maintain the TDA-water mixture in a liquid state for an extended time period, and thus, allow for storage and/or transportation of the mixture.
The present invention provides TDA formulations which can be stored or transported in liquid form :at a temperature below 95xc2x0 C. without solids settling out.
A further object is to modify the process for the production of TDI by nitration of toluene to yield DNT, hydrogenation of DNT to yield TDA and water, and reaction of dried TDA with phosgene to yield TDI in such a manner that it is possible to transport the TDA in tanker containers or as bulk goods from a first production plant to a second production plant located some distance away, without the disadvantages mentioned above.
This object is achieved by the mixtures and process of the present invention.
It has now, surprisingly, been shown that specific mixtures of TDA and water, having a water content of 1-40%, preferably 2-10%, have clearly depressed melting points and are thus substantially easier to handle and are more cost-effective to transport over long distances as bulk goods in tanker ships than is pure TDA. Tanker ships designed for transporting chemicals are generally able to maintain the goods which are being transported at temperatures in the range of about 65 to 70xc2x0 C. during transportation, and also to unload the chemicals over a similar temperature range. Thus, one object of the process according to the invention is to obtain TDA/water mixtures whose melt characteristics enable their transportation as bulk goods in conventional tanker ships.
Mixtures in accordance with the invention may be produced, in principle, by mixing pure TDA, i.e. conventional industrial isomer mixtures containing about 80 wt. % of 2,4 TDA and about 20 wt. % of 2,6-TDA, with water. These mixtures are more readily accessible, however, if during the distillation step of the hydrogenation product which contains TDA and water, the industrial process is interrupted at a suitable point such that an intermediate mixture of TDA and water, having a water content of about 1 to 40% by weight, is obtained instead of anhydrous TDA as in conventional processes.
Thus, the present invention provides a process for the production of toluylene diisocyanate comprising reacting toluene with nitric acid to yield dinitrotoluene, hydrogenating the dinitrotoluene to yield a crude solution of toluylene diamine and reaction water, and distilling the crude solution of toluylene diamine and reaction water to form an intermediate mixture of toluylene diamine and water which contains from about 1 to 40%, preferably about 2 to 10% by weight of water, then transporting this intermediate mixture of toluylene diamine and water from a first production plant to a second production plant, followed by distilling the intermediate mixture of toluylene diamine and water completely at the second production plant to yield dry toluylene diamine, and phosgenating the dry toluylene diamine to yield toluylene diisocyanate.
In another embodiment, the dinitrotoluene is hydrogenated in the presence of a solvent or diluent. This solvent or diluent may be either completely or partially removed or separated from the crude solution of toluylene diamine and reaction water in an additional step prior to distilling the crude solution of toluylene diamine and reaction water to yield the intermediate mixture of toluylene diamine and water which is suitable for storage and/or transportation.
In another embodiment, the dried toluylene diamine, after being completely distilled at a second production plant, may be subjected to further processing stages prior to phosgenation to yield toluylene diisocyanate in a manner known per se.